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“Orphan Planets” Without Solar Systems May Be More Numerous Than Stars

While we tend to think of planets as orbiting stars, as in our own solar system, according to a recent survey of the Milky Way galaxy, the findings of which were published in the latest issue of Nature [paywalled], the universe may be abundant with rogue planets that drift alone through space, with no central star.

The astronomers behind the survey discovered ten so-called “orphan planets” roughly the size of Jupiter at the heart of the Milky Way. But what’s more interesting than the planets they discovered are the implications of their discovery: As the planets were discovered within a relatively small swath of the galaxy, it’s likely, based on their ‘population density,’ that free-floating planets outnumber the stars.


The discovery indicates there are many more free-floating Jupiter-mass planets that can’t be seen. The team estimates there are about twice as many of them as stars. In addition, these worlds are thought to be at least as common as planets that orbit stars. This would add up to hundreds of billions of lone planets in our Milky Way galaxy alone.

“Our survey is like a population census,” said David Bennett, a NASA and National Science Foundation-funded co-author of the study from the University of Notre Dame in South Bend, Ind. “We sampled a portion of the galaxy, and based on these data, can estimate overall numbers in the galaxy.”

The survey is not sensitive to planets smaller than Jupiter and Saturn, but theories suggest lower-mass planets like Earth should be ejected from their stars more often. As a result, they are thought to be more common than free-floating Jupiters.

Orphan planets were already known to exist before the survey, but this is a greater density of them than was previously expected. As such, this has potentially big implications for the field of astronomy, although actually finding most of those orphan planets may be beyond the means of current observation methods and technology: Nowadays, scientists detect planets primarily by way of their gravitational interactions with nearby stars, and while they’re currently able to find larger planets, like the Jupiter-sized planets found in this survey, and smaller planets which are close to stars, small planets drifting alone through space have too minute a gravitational effect on stars to be easily discovered.

(via NASA)

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